1. Field of the Invention
The present invention relates to an image forming apparatus which uses electrophotography or electrostatic recording, such as a copying machine or a printer.
2. Related Background Art
Various image forming apparatuses, so-called tandem type color image forming apparatuses, have been proposed which superpose toner images in different colors formed by a plurality of image forming units (image forming stations) on recording material to form a color image.
FIG. 20 is a partially schematic view of such a color image forming apparatus. In the figure, the symbols SY, SM, SC, and SB represent first through fourth image forming stations disposed from right to left. The first through fourth image forming stations form a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image, respectively.
The first through fourth image forming stations SY, SM, SC, and SB are electrophotography mechanisms. The stations have electrophotographic photosensitive members 1Y, 1M, 1C, and 1B as image bearers; charging units 2Y, 2M, 2C, and 2B; developing units 4Y, 4M, 4C, and 4B; cleaning units 6Y, 6M, 6C, and 6B; and so on. The electrophotographic photosensitive members 1Y, 1M, 1C, and 1B are rotated clockwise as indicated by an arrow at a predetermined process speed (circumferential speed) to form a toner image on their surfaces by charging; exposing LY, LM, LC, and LB; and developing.
The exposing units, not shown, are laser beam scanners, for example. The laser beam scanners comprise a semiconductor laser, a polygon mirror, an F-xcex8 lens, and so on. To expose the uniformly charged surface of rotating photosensitive drum to a laser beam by scanning, the scanners emit the laser beam. This beam is modulated according to a time-series electric digital image signal carrying intended image information fed from a host apparatus (not shown) including manuscript reader having photoelectric conversion elements, such as CCD, a computer, a word processor, etc. Exposure by laser beam scanning forms on the surface of the rotating photosensitive drum an electrostatic latent image according to the intended image information.
The exposure LY in the first image forming station SY corresponds to the cyan component of a full-colored image. The developing unit 4Y uses a developer containing a cyan toner.
The exposure LM in the second image forming station SM corresponds to the magenta component of a full-colored image. The developing unit 4M uses a developer containing a magenta toner.
The exposure LC in the third image forming station SC corresponds to the yellow component of a full-colored image. The developing unit 4C uses a developer containing a yellow toner.
The exposure LB in the fourth image forming station SB corresponds to the black component of a full-colored image. The developing unit 4B uses a developer containing a black toner.
Reference numeral 51 denotes an endless belt type recording medium carrier (hereinafter called a transfer belt). The transfer belt 51 is almost horizontally tensioned under the image forming stations SY, SM, SC, and SB, across the image forming stations, and between a drive roller 52 and a turn roller 53. A drive motor 54 drives the drive roller 52, so that the transfer belt 51 turns counterclockwise as indicated by an arrow at a predetermined process speed.
The transfer belt 51 is made of a sheet of dielectric resin, such as polyethylene telephthalate (PET), polyvinylidene fluoride, or polyurethane. The belt is endless, that is, its original ends are connected together, or it is seamless.
Reference numerals 55Y, 55M, 55C, and 55B are transfer charging blades. The blades are disposed inside the transfer belt 51, with the upper half of the transfer belt 51 pressed against the underside of the photosensitive members 1Y, 1M, 1C, and 1B in the image forming stations SY, SM, SC, and SB. In the image forming stations, SY, SM, SC, and SB, the contacts between the underside of the photosensitive members 1Y, 1M, 1C, and 1B and the transfer belt 51 provides first through fourth transfer units (transfer nips) TY, TM, TC, and TB.
Reference numeral 56 is a cleaning unit for the transfer belt 51 (transfer belt cleaner). The transfer belt cleaner presses a cleaning web (non-woven fabric) 57 against the transfer belt 51 to remove residual toner and other foreign material from the transfer belt.
A sheet feeding apparatus, not shown, feeds recording material P (material onto which an image is transferred), and a registration roller 8 places the material P on the upper half of the transfer belt 51 from the side of the first image forming station SY at a predetermined control timing.
The recording material P on the transfer belt 51 is electrostatically attracted to the belt 51. As the transfer belt 51 turns, the material is conveyed from the first transfer unit TY to the second transfer unit TM, to the third transfer unit TC, to the fourth transfer unit TB. Images are transferred onto the material P as follows:
1) The first transfer unit TY transfers a cyan toner image formed on the surface of the photosensitive member 1Y in the first image forming station SY.
2) The second transfer unit TM transfers a magenta toner image formed on the surface of the photosensitive member 1M in the second image forming station SM.
3) The third transfer unit TC transfers a yellow toner image formed on the surface of the photosensitive member 1C in the third image forming station SC.
4) The fourth transfer unit TB transfers a black toner image formed on the surface of the photosensitive member 1B in the fourth image forming station SB.
By transferring and superposing these four toner images, a toner image corresponding to an intended full-colored image is composited on the material P.
Toner images start to be formed in synchronism with each other at the first through fourth image forming stations SY, SM, SC, and SB. Thus the toner images formed at the image forming stations are transferred onto recording material P, which is conveyed on the transfer belt 51, so that the images are positioned and superposed as predetermined.
After the recording material P is conveyed on the belt through the fourth transfer unit TB, electricity is discharged from the recording material P by the electricity discharging apparatus 9. Then the material is separated from the transfer belt 51 and introduced into a thermal fixing apparatus, not shown. The apparatus fixes an unfixed toner image on the recording material as a permanent image by melting and color mixing. Finally, the material, which bears a fixed image, is discharged.
After image transfer, residual toner on the photosensitive members 1Y, 1M, 1C, and 1B in the image forming stations SY, SM, SC, and SB is removed by the cleaning units 6Y, 6M, 6C, and 6B.
Toner and other foreign material deposited on the transfer belt 5 is removed by the cleaning unit 56.
Besides color image forming apparatuses which have a plurality of image bearers 1Y, 1M, 1C, and 1B as described above, a color image forming apparatus is available which repeats a process consisting of charging, latent-image formation, development, and cleaning, using one image bearer to form a color image. Some of these color image forming apparatuses are of a type which attracts recording material to a transfer belt or a transfer drum to superpose toner images one after another on the same recording material and of an intermediate transfer type which superposes a plurality of toner images on an intermediate transfer member and transfers the all superposed images onto the recording material.
For such an electrophotographic apparatus, a cleanerless type, in which the cleaner for cleaning the residual toner on the photosensitive member after image transfer is eliminated and a developing device collects and reuses the residual toner, is contemplated.
When an image forming apparatus of a simultaneous development/cleaning type (cleanerless process type) repeats image formation, the preceding image is slightly left, that is, a so-called positive ghost occurs, because residual toner cannot be collected by the developing device completely.
If an image bearer part under the residual toner cannot be charged when the residual toner on the image bearer passes a charging member, fog removal potential (Vback) which is sufficient to collect residual toner using a developing device cannot occur in the image bearer part. This phenomenon causes a positive ghost.
If a contact type charging member is used, the contact charger is contaminated, thus resulting in a marked positive ghost.
To prevent a positive ghost, Japanese Patent Application Laid-Open No. 10-31346 proposes a method for properly developing an image and collecting toner at the same time. The method consists of the following sequential steps: (1) collecting residual toner in a contact type charging member, (2) giving a regular charging polarity to the collected toner by friction between the toner and the contact type charging member to form an electrostatic latent image on an image bearer, and (3) discharging the collected toner onto a photosensitive member.
Although the method is used, transfer efficiency may significantly decrease due to, for example, continuous formation of images with a high image ratio, temperature, humidity, the type of paper, etc. In such a case, the amount of collected toner markedly increases, thus preventing discharged toner from being collected completely at a location of development. Discharged toner which passes the location shows up as smears on recording material.
By analogy with Japanese Patent Application Laid-Open No. 6-51672, collected toner could be intentionally discharged onto that portion of the surface of a photosensitive member which paper does not pass to prevent smears from occurring on recording material. Indeed, this method reduces smears if discharged toner cannot be collected completely.
For example, if transfer efficiency significantly decreases due to continuous formation of images with a high image ratio, temperature, humidity, the type of paper, etc., toner collected by a contact type charging member and then discharged is recollected after it passes locations of development and transfer. Thus the amount of residual toner on an image bearer increases. Accordingly, the amount of toner collected by the contact type charging member also increases. Because the residual toner attaches to the contact type charging member and enters it when collected by the charging member, the electric resistance of the contact type charging considerably changes. For example, if the contact type charging member is a magnetic brush charger (injection charger), toner enters the magnetic brush, thus increasing its electrical resistance. As a result, an enough charge does not transfer when the brush passes a charging nip. This causes the potential of the surface of the photosensitive member to be lower than a voltage applied after the brush passes the charging nip. A potential difference xcex94V between the potential of the surface of the photosensitive member and the applied voltage increases with the amount of toner entering the magnetic brush, so that fog occurs in a developing portion.
A problem with the color image forming apparatuses is that in a tandem system or other system, when another color is to be transferred, the toner image once transferred onto the recording material or intermediate transfer member is transferred again onto the image bearer (hereinafter referred to as xe2x80x9cretransferxe2x80x9d), so that a desirable toner image cannot be obtained.
Considering that all color image forming apparatuses reproduce all colors by superposing chromatic colors, retransfer has an effect on all chromatic colors superposed on the entire recording material.
In a tandem type color forming apparatus which incorporates a cleanerless process, residual toner and retransferred toner are collected using a fog removal bias Vback during development. Because retransferred toner differs in color from residual toner, a developer causes color mixing when retransferred toner is recollected during development as is residual toner. Toners in different colors are accumulated in a developing device as image formation proceeds, so that desired colors cannot be obtained. This phenomenon is remarkable if a large amount of toner is retransferred.
For a contact charging type transfer image forming apparatus which uses a cleanerless process, if residual toner attaches to a contact charging member or enters the member when collected by the member, the electrical resistance of the member changes. For example, if the contact charging member is a magnetic-brush charger (injection charger), toner enters the magnetic brush, so that its electrical resistance gradually increases. Thus while a photosensitive member passes a charging nip, an enough charge does not move. Consequently, the photosensitive-member surface potential is lower than an applied voltage. The larger the amount of toner entering the magnetic brush, the larger the difference xcex94V between the photosensitive-member surface potential and the applied voltage. As a result, fog occurs in a developing portion. To prevent this problem, the amount of toner entering the magnetic brush must be kept equal to or less than a certain value.
If toner introduced into the magnetic-brush charger is given a charge with the same polarity as the photosensitive-member potential by contact with magnetic-brush carriers (magnetic particles and charging carriers), an electric field produced by the difference xcex94V discharges toner from the magnetic brush onto the photosensitive member. As disclosed in U.S. Pat. No. 5,835,821, a method is known which, when no image is formed, reduces the amplitude Vpp of the AC component of a charging bias or stops application of the AC component, using development to increase the difference xcex94V, thus promoting toner discharge to inhibit the electrical resistance of the magnetic brush from increasing.
Discharging toner between sheet transfers or in a post-rotation process after image formation is completed allows the amount of toner entering the magnetic brush to be kept equal to or less than a certain value for prolonged periods of time.
A problem with multiple-transfer image forming apparatuses is that discharged toner which is transferred onto the transfer belt in an upstream image forming unit is retransferred onto the photosensitive drum in a downstream image forming unit and collected by the charging portion of the downstream image forming unit, thus resulting in deteriorated charging performance. The inventors found that the amount of the retransferred toner increases with that of the discharged toner.
It is an object of the present invention to provide an image forming apparatus which keeps good charging performance for prolonged periods of time.
It is another object of the present invention to provide an image forming apparatus which collects and discharges toner using a charger.
It is still another object of the present invention to provide an image forming apparatus which prevents toner discharged from a charger from adversely affecting an image.
It is further object of the present invention to provide an image forming apparatus comprising:
an image carrier which carries an electrostatic image;
developing means for developing the electrostatic image using toner;
a rotating transfer member which transfers a toner image on the image bearer;
charging means for charging the image bearer on which residual toner is deposited after transfer by the rotating transfer member, the charging means allowing the residual toner on the image bearer to be collected; and
returning means for returning the toner in the charging means to the image bearer,
in which the rotating transfer member transferring onto the rotating transfer member at least a portion of the toner returned to the image bearer by the returning means.
Other objects of the present invention will be clear from the following descriptions.